“…However, there is a considerable amount of controversy in the literature concerning the assignment of the CO stretch in adsorbed CO on the surface of copper in its three oxidation states: Cu(0)−Cu metal, Cu(I)−Cu 2 O, and Cu(II)−CuO. − In addition to the great variance in values for the CO frequency of CO adsorbed on oxidized and reduced copper, it has also been observed that the frequency of adsorbed CO is highly sensitive to surface structure. , Figure displays a compilation of band frequencies and assignments from the literature for CO adsorbed on different copper surfaces. − It is fairly well agreed upon that IR bands between ∼2065 and 2110 cm -1 are indicative of CO adsorbed on reduced copper particles. − ,, However, the region between 2110 and 2200 cm -1 is not as clearly defined. In some cases, bands in the 2115−2130 cm -1 region have been assigned to CO adsorbed on Cu 2 O/SiO 2 38,40,42 and bands in the 2125−2200 cm -1 region to CO adsorbed on CuO/SiO 2 . ,− In other cases, bands near 2130 cm -1 have been assigned to CO adsorbed on atomically rough, reduced-copper particles. , It has also been suggested that it is not the location of the CO absorption band which is the critical factor, but the stability of the species under vacuum at room temperature and the formation of CO 2 or surface carbonate from reaction with surface oxygen which indicates whether the sample is oxidized or reduced. , 1 A compilation of the wide range of values reported in the literature for the frequency and assignment of CO adsorbed on various copper surfaces that differ in surface morphology and oxidation state. There is significant overlap in the frequency range for CO adsorbed on reduced and oxidized copper making it difficult to use the frequency of the CO absorption band as the only measure of the oxidation state of the copper surface atoms.…”
Transmission infrared spectroscopy and temperature programmed
desorption have been used to investigate
the chemistry of CH3I adsorbed on silica-supported copper
nanoparticles. The following three factors affect the
chemistry of CH3I on Cu/SiO2: (i) the
oxidation state of the copper, (ii) the hydroxyl group coverage on the
silica
support, and (iii) the surface roughness of the copper particles.
These three factors can be controlled by sample
preparation. On reduced-Cu/SiO2 samples, C−I bond
dissociation in adsorbed methyl iodide results in the
formation
of adsorbed methyl groups on the copper surface. The frequency of
the symmetric stretch of adsorbed methyl groups
is at 2913 cm-1 for copper nanoparticles with
atomically smooth surface morphology and at 2924 cm-1 for
copper
nanoparticles with atomically rough surface morphology. In
addition to methyl groups adsorbed on the copper
nanoparticles, for reduced Cu/SiO2 samples with Si−OH
groups present in close proximity to the copper particles,
methyl groups can spill over on to the silica support and react with OH
groups to form SiOCH3. The silica
hydroxyl
coverage also plays a role in methyl reactions on the copper particles
as hydroxyl groups provide a source of hydrogen
atoms. Methane and ethane are the predominant reaction products
for reduced Cu/SiO2 samples with high hydroxyl
group coverage whereas methane, ethane, and ethylene form on samples
with low silica hydroxyl group coverage.
The copper particle morphology may also play a role in the
chemistry of adsorbed methyl groups as there is some
evidence for slower methyl reaction kinetics on the copper
nanoparticles with rough surface morphology. C−I
bond dissociation in adsorbed methyl iodide occurs on oxidized-copper
nanoparticles as well. The infrared spectrum
taken after adsorption of CH3I on
oxidized-Cu/SiO2 is consistent with the presence of
adsorbed methoxy groups and
bidentate formate on the oxidized-copper particles. The chemistry
of methyl groups on oxidized-copper particles is
similar to that of methanol on oxidized-copper particles. Finally,
the use and complexities of characterizing these
samples and copper catalysts in general with CO adsorption in
conjunction with infrared spectroscopy are discussed.
“…However, there is a considerable amount of controversy in the literature concerning the assignment of the CO stretch in adsorbed CO on the surface of copper in its three oxidation states: Cu(0)−Cu metal, Cu(I)−Cu 2 O, and Cu(II)−CuO. − In addition to the great variance in values for the CO frequency of CO adsorbed on oxidized and reduced copper, it has also been observed that the frequency of adsorbed CO is highly sensitive to surface structure. , Figure displays a compilation of band frequencies and assignments from the literature for CO adsorbed on different copper surfaces. − It is fairly well agreed upon that IR bands between ∼2065 and 2110 cm -1 are indicative of CO adsorbed on reduced copper particles. − ,, However, the region between 2110 and 2200 cm -1 is not as clearly defined. In some cases, bands in the 2115−2130 cm -1 region have been assigned to CO adsorbed on Cu 2 O/SiO 2 38,40,42 and bands in the 2125−2200 cm -1 region to CO adsorbed on CuO/SiO 2 . ,− In other cases, bands near 2130 cm -1 have been assigned to CO adsorbed on atomically rough, reduced-copper particles. , It has also been suggested that it is not the location of the CO absorption band which is the critical factor, but the stability of the species under vacuum at room temperature and the formation of CO 2 or surface carbonate from reaction with surface oxygen which indicates whether the sample is oxidized or reduced. , 1 A compilation of the wide range of values reported in the literature for the frequency and assignment of CO adsorbed on various copper surfaces that differ in surface morphology and oxidation state. There is significant overlap in the frequency range for CO adsorbed on reduced and oxidized copper making it difficult to use the frequency of the CO absorption band as the only measure of the oxidation state of the copper surface atoms.…”
Transmission infrared spectroscopy and temperature programmed
desorption have been used to investigate
the chemistry of CH3I adsorbed on silica-supported copper
nanoparticles. The following three factors affect the
chemistry of CH3I on Cu/SiO2: (i) the
oxidation state of the copper, (ii) the hydroxyl group coverage on the
silica
support, and (iii) the surface roughness of the copper particles.
These three factors can be controlled by sample
preparation. On reduced-Cu/SiO2 samples, C−I bond
dissociation in adsorbed methyl iodide results in the
formation
of adsorbed methyl groups on the copper surface. The frequency of
the symmetric stretch of adsorbed methyl groups
is at 2913 cm-1 for copper nanoparticles with
atomically smooth surface morphology and at 2924 cm-1 for
copper
nanoparticles with atomically rough surface morphology. In
addition to methyl groups adsorbed on the copper
nanoparticles, for reduced Cu/SiO2 samples with Si−OH
groups present in close proximity to the copper particles,
methyl groups can spill over on to the silica support and react with OH
groups to form SiOCH3. The silica
hydroxyl
coverage also plays a role in methyl reactions on the copper particles
as hydroxyl groups provide a source of hydrogen
atoms. Methane and ethane are the predominant reaction products
for reduced Cu/SiO2 samples with high hydroxyl
group coverage whereas methane, ethane, and ethylene form on samples
with low silica hydroxyl group coverage.
The copper particle morphology may also play a role in the
chemistry of adsorbed methyl groups as there is some
evidence for slower methyl reaction kinetics on the copper
nanoparticles with rough surface morphology. C−I
bond dissociation in adsorbed methyl iodide occurs on oxidized-copper
nanoparticles as well. The infrared spectrum
taken after adsorption of CH3I on
oxidized-Cu/SiO2 is consistent with the presence of
adsorbed methoxy groups and
bidentate formate on the oxidized-copper particles. The chemistry
of methyl groups on oxidized-copper particles is
similar to that of methanol on oxidized-copper particles. Finally,
the use and complexities of characterizing these
samples and copper catalysts in general with CO adsorption in
conjunction with infrared spectroscopy are discussed.
“…It is assumed that the reaction on CuO takes place between adsorbed CO molecules and lattice oxygen of CuO, which is replenished by gas-phase O 2 . 12,14,15 Therefore, the amount of lattice oxygen of CuO should be related to its reactivity. In other words, more lattice oxygen from CuO is available for CO oxidation in the preoxidized Cu catalyst than in the CuO shell− Cu 2 O core of the prereduced Cu catalyst.…”
The oxidation state of Cu nanoparticles
during CO oxidation in
CO + O2 gas mixtures was sensitively monitored via localized
surface plasmon resonances. A microreactor, equipped with in situ
UV–vis and mass spectrometry, was developed and used for the
measurements. Cu nanoparticles of ∼30 nm average diameter were
supported on optically transparent, planar quartz wafers. The aim
of the study is 2-fold: (i) to demonstrate the performance and usefulness
of the setup and (ii) to use the combined strength of model catalysts
and in situ measurements to investigate the correlation between the
catalyst oxidation state and its reactivity. Metallic Cu is significantly
more active than both Cu(I) and Cu(II) oxides. The metallic Cu phase
is only maintained under conditions where close to full oxygen conversion
is achieved. This implies that kinetic measurements, aimed at determining
the apparent activation energy for metallic Cu under realistic steady-state
conditions, are difficult or impossible to perform.
~~~ ~ FTIR spectra are reported of CO adsorbed on sil ica-supported copper catalysts prepared from copper(i1) acetate monohydrate. Fully oxidised catalyst gave bands due to CO on CuO, isolated Cu2+ cations on silica and anion vacancy sites in CuO. The highly dispersed CuO aggregated on reduction to metal particles which gave bands due to adsorbed CO characteristic of both low-index exposed planes and stepped sites on high-index planes. Partial surface oxidation with N, O or H, O generated Cu' adsorption sites which were slowly reduced to Cu" by CO at 300 K. Surface carbonate initially formed from CO was also slowly depleted with time with the. generation of CO,. The results are consistent with adsorbed carbonate being an intermediate in the water-gas shift reaction of H,O and CO to H, and CO,.
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